Helicopters generally incorporate at least two rotors into their design. The large rotor providing thrust in the vertical direction is known as the main rotor. In addition to this main rotor, the traditional helicopter design incorporates a tail rotor system to counteract the torque from the main rotor system. Although operable helicopter designs have been produced without the traditional tail rotor geometry, the vast majority of helicopters use this design. The number of blades in the tail rotor itself will depend on the requirements of a particular application.
In certain tail rotor designs, the thrust developed by the tail rotor is a function of the pitch of the rotor blades. Accordingly, many such designs incorporate variable-pitch rotors so as to adjust the tail rotor thrust in flight. Most such designs have traditionally employed some form of bushings or bearings about which each of the rotor blades pivots. Typical examples of such bearings include elastomeric, PTFE-lined, and grease-lubricated bearings. Generally such designs incorporate a separate tail rotor blade and tail rotor hub.
Because of the loading on those bearings and because of the type of materials used in their construction, there can be a considerable degree of static friction, or xe2x80x9cstictionxe2x80x9d associated with such bearings. This stiction can make fine control of the rotor pitch difficult or impossible in many situations, as the pilot must place considerable force on the pitch controls in order to overcome the static friction. This degree of force will often cause overshoot of the desired blade pitch, so that additional adjustment is required.
As the tail rotor is designed to counteract the induced torque of the main rotor, the pitch of the tail rotor blades must be adjusted whenever the torque on the main rotor is adjusted, in order to ensure that the aircraft remains xe2x80x9ctrimmedxe2x80x9d. The repeated adjustment and readjustment of the blade pitch induces undesirable xe2x80x9cpilot induced oscillationsxe2x80x9d, or xe2x80x9cPIOs.xe2x80x9d These oscillations are known to compromise the integrity of the aircraft""s flight, and may result in potentially hazardous situations.
Additionally, there are a number of failure modes associated with the use of traditional tail rotor bearings. It has been found that a failure in the primary load path, in which the centrifugal force acts, can cause a catastrophic failure of one or more components in the tail rotor, including bearings, bearing support structures, or the rotor yoke itself. It has also been found that such catastrophic failure can occur without any warning to the pilot during flight, and that such failure can occur without any discernable sign of impending failure, such as could be discovered by close inspection. Additionally, it has been found that the reasonable life expectancy of the tail rotor bearings is generally between 200 to 1,000 hours, such that these components represent a considerable maintenance burden.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
The present invention relates to a composite rotor blade and method of manufacture of the same. Although the rotor blade of the present invention is described in connection with the tail rotor of a helicopter, it will be understood by those of skill in the art that the inventive concepts embodied herein are applicable to a wide variety of applicable contexts and should not be considered limited to the specific applications described herein.
The present invention relates to a composite tail rotor blade that provides higher aerodynamic performance, provides damage tolerant design with extended life expectancy, and reduced maintenance burden due to the use of composite materials. In various embodiments, the design uses a composite twist strap flexure to accommodate collective pitch control, integral with each blade.
As described above, traditional rotor blades differ from the design of the embodiment disclosed herein in that they rely on rotary bearings between the yokes and the blades to accommodate the pitch change motion of the blades relative to the yokes. In the embodiments of the present invention disclosed herein, the blades are fixed to the yokes, which are in turn fixed to the rotor mast, with no provision within the hub assembly for accommodation of rotor blade pitch. Each rotor blade incorporates an integral flexing strap, which replaces the functionality of the bearings found in traditional prior art designs by flexing about the lengthwise axis of rotor blade, so as to allow for adjustment of the pitch of the rotor blade without pitch change bearings.
The present invention achieves a reduction or elimination of static friction and increased service life of the tail rotor mechanisms through the replacement of the rotary bearings with the flexural members. In certain embodiments, the present invention also achieves a reduction or elimination of catastrophic failure modes by the incorporation of redundant load paths within the rotor structure. In certain embodiments, the tail rotor of the present invention may be employed in a xe2x80x9cpusherxe2x80x9d implementation for improved aerodynamic performance by minimizing vertical fin blockage effects.
In certain embodiments, the rotor blade of the present invention reduces the likelihood of failure at the part edge due to a novel and unique fiber placement lay-up technique wherein the reinforcement fibers within each composite belt are aligned with, and follow, the outside edges of the part. With this design, few or none of the reinforcement fibers run off the edge of the part, but rather follow the contours of the part and direct the stress through the structure of the part in a desirable manner.
The teachings of the present invention may provide improved aerodynamic efficiency, higher maneuvering capability, improved mechanical flaw tolerance design and extended life expectancy. In one embodiment, a tail rotor constructed according to the present invention has been designed to achieve a minimum life of 10,000 hours of severe duty use in ground-air-ground maneuvers, air combat maneuvers, and high cycle vibratory loads, with little or no maintenance.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.